1. Field of the Invention
The present invention relates to a freezing apparatus for substantially instantaneously freezing biological tissue samples for microscopic examination of such specimens.
2. Description of the Prior Art
Metal blocks cooled with liquid nitrogen or liquid helium have been used to freeze tissue samples for electron microscopy. In prior methods, a tissue sample has been placed on the end of a supporting rod and dropped onto a freezing block, which also served as a stop for the rod. Measures were taken to try to prevent the tissue from rebounding from the freezing surface after its initial contact. Even a millisecond bounce retards the rate of freezing of the sample sufficiently to cause ice crystal formation in the sample.
U.S. Pat. No. 4,563,883 illustrates an immersing device that has an injector which immerses a specimen in a cryogenic cooling liquid at a predetermined velocity. The injector is rotated before the vertical movement ends, to promote heat transfer from the specimen. This requires complete immersion, while the present system utilizes a cold sapphire plate which the specimen contacts for immediate freezing. U.S. Pat. No. 4,489,569 also shows a system for immersing a biological specimen in a cryogen for freezing. This, too, uses a submersion technique, including a plunger that is slidably mounted above the cryogen, and which can be manually depressed for injecting a sample and a sample holder into the cryogen.
After the samples have been frozen, they are subsequently treated for light or electron microscopic examination.